Though there is not a lot of information about paracosms and imaginary friends out there, I have found a few books, which I cherish deeply. Another source of information about paracosms and imaginary friends is scholarly journals. I have found some scholarly articles, mostly in psychological journals, to summarize and share with you. This bibliography is a work in progress, added to as I find more material.
Hoff, Eva V. 2004. "A friend living inside of me: The forms and functions of imaginary companions." Imagination, Cognition and Personality 24(2):151-189. 26 children and their imaginary companions were studied in detail. Companions were mostly children of the same age, though there were some fantasy creatures. Inspirations were varied, though mostly from friends and siblings. Imaginary friends served several purposes for their creators: inner mentors, sources of comfort, self-regulation devices and life enrichment.
Kastenbaum, R; Fox, L. 2007. "Do imaginary companions die? An exploratory study." Omega (Westport) 56(2):123-152. Adults were interviewed about the "end" of their imaginary characters’ lives. While most reported that their imaginary characters just faded away or disappeared, some reported that their imaginary characters died. The authors suggest that, at the age when kids create imaginary characters, they are also trying to figure out the status of "alive" and "dead."
Mills, Antonia. 2003. "Are children with imaginary playmates and children said to remember previous lives cross-culturally comparable categories?" Transcultural Psychiatry 40(1):62-90. 15 U.S. children with imaginary companions are compared to 15 children from India who say they remember their past lives to see whether the phenomena are cross-culturally comparable. In conclusion, yes, they seem to be similar phenomena springing from the same source.
Sawa, T; et al. 2004. "Role of imaginary companions in promoting the psychotherapeutic process." Psychiatry and Clinical Neuroscience 58(2):145-151. While usually studied as a phenomenon of childhood, imaginary companions may also manifest when a person has a psychiatric disorder. The authors point out that indulging and engaging the imaginary companion in the therapeutic process may help the therapist reach an otherwise recalcitrant patient.
Seiffge-Krenke, Inge. 1997. "Imaginary companions in adolescence: sign of a deficient or positive development?" Journal of Adolescence 20(2):137-154. 241 teens between 12 and 17 who had imaginary companions were surveyed about the traits and relationships of their imaginary friends. Three hypotheses were tested: 1) that only kids with social failings create imaginary friends; 2) that gifted, really creative kids create imaginary friends; and 3) that narcissistic kids create imaginary friends to feed their need for ego boosting. In conclusion, the creators of imaginary friends were socially and creatively competent teens.